Edexcel iGCSE Biology-6.5 Yeast in Food Production- Study Notes- New Syllabus

Role of Yeast in Food Production (Including Bread)

📝 Introduction

• Yeast is a type of single-celled fungus used in food production.
• It is widely used in making bread, cakes, and other baked foods.
• Yeast is important because it carries out fermentation, which produces carbon dioxide and alcohol.
• In bread making, the gas produced helps the dough to rise and become soft and spongy.
• Yeast works best in warm, moist conditions with food (sugar).

🍞 What is Yeast?

Definition

• Yeast is a living microorganism (fungus) that respires to obtain energy.
• It feeds on sugars (like glucose) present in dough.

Conditions Needed for Yeast Activity

Yeast needs certain conditions to work properly:

• Warm temperature (around 30–40°C) → speeds up respiration.
• Moist environment → helps chemical reactions occur.
• Sugar (glucose or starch broken into sugars) → food for respiration.
• Limited oxygen (for fermentation) → allows anaerobic respiration.

⚙️ Role of Yeast in Bread Making

Yeast is used to make bread dough rise through a process called fermentation.

Step 1: Mixing Ingredients

• Flour, water, yeast, and sugar are mixed to form dough.
• The dough contains food (sugars) for yeast.

Step 2: Respiration by Yeast

Yeast begins to break down sugar to release energy.

There are two types of respiration:

Aerobic Respiration (with oxygen)

• Happens when oxygen is available.
• Produces carbon dioxide, water, and energy.
• Less important in bread making.

Anaerobic Respiration (Fermentation)

• Happens when oxygen is low inside dough.
• Yeast breaks down sugar without oxygen.
• Word equation:

Glucose → Carbon dioxide + Ethanol + Energy

Step 3: Production of Carbon Dioxide

• Carbon dioxide gas is released by yeast during respiration.
• The gas forms bubbles inside the dough.
• These bubbles get trapped in the sticky dough.

Step 4: Dough Rising

• Gas bubbles expand inside the dough.
• The dough becomes larger and rises.
• The structure becomes soft and spongy.

Step 5: Baking the Bread

• Heat kills the yeast.
• Carbon dioxide bubbles expand further in the oven.
• Ethanol evaporates during baking.
• The final bread becomes light and airy.

🌟 Why Yeast is Important in Bread Making

• Makes bread soft and fluffy.
• Improves texture and volume of bread.
• Produces the characteristic light structure of baked bread.
• Helps create holes inside bread (air spaces).

📊 Summary of Yeast Action in Bread

⚠️ Factors Affecting Yeast Activity

Temperature

• Too cold → yeast works slowly.
• Warm (30–40°C) → yeast works fastest.
• Too hot → yeast dies.

Sugar Availability

• More sugar → more respiration → faster gas production.
• No sugar → yeast cannot produce energy.

Water

• Needed for yeast to become active.
• Helps enzymes and reactions occur.

Oxygen

• Small amount helps initial growth.
• Lack of oxygen leads to fermentation (important for bread rising).

🧠 Other Roles of Yeast in Food Production

• Alcoholic beverages: Ferments sugars to produce ethanol
• Fermented foods: Enhances flavour and texture
• Nutritional benefits: Yeast adds vitamins (B-complex) and protein

📊 Summary Table

💡 Quick Recap 
Yeast = unicellular fungus → ferments sugars
Sugar → CO₂ + Ethanol → energy
CO₂ makes bread rise, ethanol evaporates during baking
🧠 Mnemonic: “Yeast Eats Sugar → CO₂ Up → Bread Puff”